Add new group level markdown_preview route

What does this MR do and why?

New route is properly scoped as /groups/*group_id/-/markdown_preview as described in gitlab-foss#61985 (moved)

Removal of the old route is tracked in #442218 and we can remove it one release after the frontend has completely stopped using it.

This came up in #442096 (comment 1776482956) as currently we need to url encode the group's fullpath for it to work properly

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Related to #442096 (closed)